docs: trim zen-free intro to one-line API key note (#690)

Drop the two-line explanation; link official OpenCode Zen signup instead of
a provider tutorial.

docs/opencode-zen-free-preset.md

@@ -3,7 +3,7 @@
 This preset runs entirely on free-tier models. Every agent uses an `opencode/...-free`
 model, so there is no usage cost.
 
-You still need an API key for the `opencode` provider, but the free models themselves are free to use. Set the preset and start.
+You need an API key for the `opencode` provider. Sign up at [OpenCode Zen](https://opencode.ai/zen) (skip billing; the free models need no balance) and connect it with `/connect`.
 
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